1. Field of the Invention
The present invention relates to a scroll-type fluid displacement apparatus and, more particularly, to a scroll-type fluid compressor having improved spiral elements on its scroll members.
2. Description of the Related Art
Scroll-type fluid displacement apparatuses are known in the prior art. For example, U.S. Pat. No. 4,678,415, issued to Hirano et al., discloses a basic construction of a scroll-type fluid displacement apparatus including two scroll members, each having an end plate and a spiroidal or involute spiral element extending from the end plates. The scroll members are maintained angularly and radially offset so that both spiral elements interfit to form a plurality of line contacts between their spiral curved surfaces to thereby seal off and define at least one pair of fluid pockets. The relative orbital motion of the two scroll members shift the line contact along the spiral curved surfaces and, as a result, change the volume in the fluid pockets. The volume of the fluid pockets increases or decreases depending on the direction of orbital motion. Thus, a scroll-type apparatus is applicable to compress, expand, or pump fluids.
A scroll-type fluid displacement apparatus is suitable for use as a refrigerant compressor. When used as a compressor, it is desirable for the scroll members to have sufficient mechanical strength to compress fluid under high pressure. In scroll members known in the prior art, the end plates and associated spiral elements are integrally formed. In those scroll-type fluid compressors, the interfitting spiral elements, normally constructed of lightweight alloys, such as an aluminum alloy, are subject to several temperature changes which are caused when fluid moves to the center of the compressor, increasing its pressure and decreasing its volumne. The hottest temperature exists in the center of the compressor, because that pocket has the smallest volume and highest pressure. This causes thermal expansion at the center of the spiral elements to be greater than at any other portion. However, the base or end portion of the spiral element, i.e., the portion which joins the end plate, particularly the inner end portion or edge, is subjected to greater stress than the outer radial portion. Accordingly, due to fatigue and deterioration caused by this stress, the strength and rigidity of the inner portion of the spiral element is substantially reduced over time. As a result, the center of the spiral element is subject to damage and failure.
Further, the scroll-type compressor is particularly suitable for use in an automobile air conditioner where compact size is desirable. However, if the height of the spiral element is increased to enlarge the displacement volume of the compressor without expanding its overall diameter, the stress developed inside the scroll is increased. Accordingly, the above described deterioration of the radial inner portion of each spiral element is hastened.
One solution to these problems is disclosed in U.S. Pat. No. 4,547,137 (Japanese Patent No. HEI-3-72839). The outer and inner side wall surfaces of both wraps define involute curves. The involute outer side wall surface starts from an arbitrary involute angle, and the involute inner side wall surface starts from an involute angle of 180 degrees from the arbitrary involute angle. The starting points of the involute side wall surfaces are interconnected by an inner end surface comprised of at least two arcuate surfaces to form a thicker inner end portion of the wrap. The inner and outer end portions of the spiral wraps, i.e., where the inner and outer involute curves start, are subjected to significant stress since those portions are in contact with the opposite spiral element during sealing and are subjected to high fluid pressure during operation. solution is disclosed in U.S. Pat. No. 4,594,061 (Japanese Utility Model Patent No. HEI-1-26315). This patent discloses that a base or proximal portion of an inner end of each spiral wrap is provided with an extension, such as a rib portion. The rib portion increases the cross-sectional area of the base or proximal portion of the spiral wrap such that it is larger than the cross-sectional area of the upper or distal portion of the wrap. Therefore, the strength of the base portion of the inner end of the wrap is greatly increased and destruction of the wrap due to high stress and high temperature is significantly reduced. However, radial sealing of the fluid pockets must be maintained in a scroll-type compressor in order to achieve efficient operation, but in this arrangement, complete engagement of the spiral wraps cannot be realized. Consequently, the compression efficiency is lowered.
These and other problems with prior art fluid development apparatuses are sought to be addressed by the following preferred embodiments.